A variety of different types of sensors are used to measure strain, vibration, pressure and/or forces. Some sensor designs are integrated onto a chip along with electronics to form intelligent sensor systems. These intelligent sensor systems are used in many applications such as internal combustion engine measurements (e.g., in-cylinder pressure and/or exhaust among others), gas turbines, aircraft engines and wings, down-hole monitoring in oil, gas and geothermal energy explorations and drilling as well as carbon capture and sequestration operations.
Historically, sensors that utilized semiconductors were made from materials such as silicon and SOI. These materials typically have limited high temperature capabilities (Si<150° C., SOI<300° C.).
Some recent designs for sensor systems incorporate wide band gap semiconductors like GaN such that the sensors and associated electronics are able to operate at temperatures above 350° C. These more recent sensor systems typically utilize GaN based strain sensors that rely on GaN HEMT, MISFET or MESFET transistors.
GaN based sensors typically operate by measuring the changes in current that flows from source to drain within the transistors. The current that flows through these types of transistors from source to drain changes because of piezoelectric charges that are generated by stress (e.g., parallel with the in-plane strain) placed on the transistors.
Other types of GaN based strain sensors monitor the piezoresistive effect in metal-semiconductor metal (MSM) structures. There are other strain sensors that utilize strain sensing nitride heterostructures.
There is a need for a sensor that is easier to read than existing sensors which include capacitive strain sensors formed with Schottky or MIS diodes. Measuring the capacitance through a diode can be more difficult than measuring a current or a voltage because signal processing is typically required when measuring capacitance whereas current and voltage can be measured directly.